Coaxial Cable

In a coaxial cable, one conductor is in the form of a tube with the second running concentrically along the axis. The inner conductor is supported by a solid dielectric or by discs of dielectric material placed at regular intervals along its length. A number of these cable are usually combined together with twisted pairs to form a multi-pair cable.

The structure of the coaxial cable ensures that, at normal operating frequencies, the electromagnetic field generated by the current flowing in it is confined to the dielectric. Radiation is therefore severely limited. At the same time, the outer conductor (normally grounded) protects the cable from extraneous signals such as noise and cross-talk.

The primary constants of the coaxial cable are much better behaved than those of the twisted pair. The inductance, L, capacitance, C, and conductance, G, per unit length are, in general, independent of frequency. The resistance, R, per unit length is a function of frequency due to skin effect; it varies as a function of vf.

The frequency characteristics of a 0.375 inch (9.5 mm) coaxial cable are shown in Figure 12.10. As expected, the coaxial cable has a much larger bandwidth than the twisted pair. However, it still requires repeaters and frequency equalizers for analog lines and phase equalization for digital signal transmission. The characteristics of the repeaters are usually adaptively controlled to correct for changes in temperature and other operating conditions.

Coaxial cable is used for transmitting data at 274.176 Mbit/s in the LD-4 (Bell-Canada) and T4M (Bell System in the USA) systems. They have 4032 voice

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